Update cross-signing status ui after activation in settings

[NicolasBuquet]

When activating cross-signing in Security settings, either directly or by activating Secure Backup, the cross-signing status is not updated in UI.

If you leave Security Settings and open it back right away, the cross-signing status is updated.

Update it right after activation without having to leave the screen.

Pull Request Checklist

• [X ] I read the contributing guide
• [ ] UI change has been tested on both light and dark themes, in portrait and landscape orientations and on iPhone and iPad simulators
• [ ] Accessibility has been taken into account.
• [X ] Pull request is based on the develop branch
• [X ] Pull request contains a changelog file in ./changelog.d
• [X ] You've made a self review of your PR
• [ ] Pull request includes screenshots or videos of UI changes
• [X ] Pull request includes a sign off

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (d50b82a) 12.37% compared to head (be4fbc3) 12.03%. Report is 25 commits behind head on develop.

Files	Patch %	Lines
...Modules/Settings/Security/SecurityViewController.m	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #7714 +/- ## =========================================== - Coverage 12.37% 12.03% -0.34% =========================================== Files 1650 1650 Lines 163869 163920 +51 Branches 67267 67277 +10 =========================================== - Hits 20277 19727 -550 - Misses 142927 143533 +606 + Partials 665 660 -5 ``` | [Flag](https://app.codecov.io/gh/vector-im/element-ios/pull/7714/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vector-im) | Coverage Δ | | |---|---|---| | [uitests](https://app.codecov.io/gh/vector-im/element-ios/pull/7714/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vector-im) | `55.03% <ø> (+0.06%)` | :arrow_up: | | [unittests](https://app.codecov.io/gh/vector-im/element-ios/pull/7714/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vector-im) | `5.86% <0.00%> (-0.36%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vector-im#carryforward-flags-in-the-pull-request-comment) to find out more.

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